Math 308 Practice Test for Final Exam Winter 2015

Transcription

1 Math 38 Practice Test for Final Exam Winter 25 No books are allowed during the exam. But you are allowed one sheet ( x 8) of handwritten notes (back and front). You may use a calculator. For TRUE/FALSE problems, you just need to cross the right box. For each correct answer, you will get point, for each incorrect answer, - point is added. For no answer you will get zero points. In each subsection of the TRUE/FALSE part, you can never get less than zero points. Only the sections that we covered in class will be relevant.!!so, if there is a problem that we is related to a topic we might not get in class, you will not need to know that for the exam!! You can earn bonus points for turning this practice test in by March 3th AND by doing the online evalution for this course. All test that are turned in by March th will be checked. In the exam there will be points for formal correctness as we have already seen in the Midterm. You will be given the chance to make up some points after the exam. Be sure to check your s frequently after the exam, so that you will be able to use that chance. I certify that I have evaluated this course online. Your Signature

2 .) (a) (b) (c) Cross the right box for the statements about linear systems. There are homogeneous systems with no solution. TRUE FALSE Every linear system with more variables than equations has TRUE FALSE at least one solution. A linear system of the form Ax = b with A an (n, m)-matrix and b a vector in R n has a solution, if and only if b is in the column space of A. Let u i be vectors in R n. Then the linear system [u u 2... u n u 2 has [,,,..., t as a solution. Any homogeneous system with 5 variables and 3 equations has infinitely many solutions. Let s R m and A an (n, m)-matrix such that As = b. Then s is a solution to Ax = b. If Ax = b has a solution and Ax = has infinitely many solutions, then Ax = b has also infinitely many solutions. Cross the right box for the statements about linear independence, span and bases. Including the zero vector in R n always gives a linearly dependent set of vectors in R n. Let {u,... u n } R m. Then span{u, u 2,... u n } {u, u + TRUE FALSE u 2, u 2, u 3,... u n } {u, u 2, u 3, u 4, u 5 } R 4 is always linearly dependent. {u, u 2, u 3, u 4, u 5 } R 4 always spans R 4. TRUE FALSE If S is a subspace of R 4 of dimension 3, then any basis of S must contain exactly 3 vectors of R 4. Every subspace has a unique basis. TRUE FALSE If {u, u 2, u 3 } spans R 3, then so does {u, u 2, u 3, u 4 }, for a vector u 4 R 3. Cross the right box for the statements about linear independence, span, bases. Let u R m and a R. Then {u, au} is linearly dependent. If {u, u 2, u 3, u 4 } spans R 4, then the set must be a basis of R 4. There cannot be a basis that contains the zero vector. The only subspace of R n with dimension n is R n. Let S, S 2 be two subspaces of R n with S S 2. Then dim(s ) dim(s 2 ). Let S be a subspace of R 7 with dim(s) = 3. Then there is a TRUE FALSE linearly independent set U S with 4 elements. The number of elements in a basis of a subspace S {} is always equal to the dimension of S. If S = span{u, u 2, u 3 }, then dim(s) = 3. TRUE FALSE Assume that S R 2 is a subspace. Then the possible dimensions of S are, and 2. S = span{[, 2 t } and S 2 = span{[, 3 t } both are onedimensional subspaces of R 2, but they are not equal.

3 (d) Cross the right box for the statements about matrices and homomorphisms. If T : R 3 R 7 is a linear homomorphism, then the corresponding TRUE FALSE matrix A T such that T (x) = A T x is a (3, 7)-matrix. If T : R 3 R 4 is a linear homomorphism with corresponding matrix A T, then ker(t ) = null(a T ). The matrix product A B for a (3, 4)-matrix A and (3, 4)- matrix B is defined. TRUE FALSE T : R 3 R 3 is injective and surjective, if and only if the corresponding matrix A T is invertible. If A is a (5, 3)-matrix, then the corresponding linear homomorphism T (x) = Ax has R 3 as domain and R 5 as codomain. Let A, B be arbitrary (n, n)-matrices. Then the following TRUE FALSE equation is always valid: (A + B) 2 = A 2 + 2A B + B 2. If T : R 7 R 7 is injective, then T is invertible. TRUE FALSE Let T : R 5 R 8 be a linear homomorphism. Then ker(t ) is a subspace of R 8. TRUE FALSE (e) Cross the right box for the statements about the determinant of an (n, n)-matrix A. If det(a) =, then A has no inverse. If A and B are invertible, then so is A B. If all the cofactors of A are nonzero, then det(a). TRUE FALSE If the columns of A are linearly independent, then det(a). The determinant is only defined for square matrices. rank(a) = n if and only if det(a). A 2 is the zero matrix if and only if A is the zero matrix. TRUE FALSE If A 2 is the zero matrix, then det(a) = (f) Cross the right box for the statements about column- and rowspace of a matrix A. If A is a square matrix, then col(a) = row(a). TRUE FALSE If A is a matrix, then dim(col(a)) = dim(row(a)). If Ax = b is a consistent system, then b is an element of col(a). Let A be a (4, 3)-matrix. Then nullity(a) could be equal to 5. TRUE FALSE (g) Cross the right box for the statements about eigenvalues and eigenspaces of a matrix A. The zero vector is an eigenvector for the eigenvalue λ =, TRUE FALSE because A =. If is an eigenvalue for A, then det(a) =. If A is a (2, 2)-matrix with the only eigenvalue, then A is TRUE FALSE the zero-matrix. It is possible for A to have no eigenvalue over R. (h) Cross the right box for the statements about orthogonality of vectors. If u u 2 = and u 2 u 3 =, then u u 3 =. TRUE FALSE If u, v both have only nonnegative entries, then u v.

4 2. Consider the following linear homomorphism: u u + 2u 2 u 3 T : u 2 u 3 4u 7u 2 + 7u 3 u u 2 + 5u 3 (a) Find the corresponding matrix A, such that T (x) = Ax for all x R (b) Find the kernel of T Therefore, ker(t ) = span{[,, t }. (c) Calculate the determinant of A. Is T invertible?. det(a) = =, 2 3 so A is invertible. (d) Find the inverse T of T using A , so the inverse is (e) Do the columns of A form a basis of R 3? Give an explanation for your answer or cite a suitable theorem. They form a basis because of the Big Theorem and (b) or (c) of this problem..

5 (f) Let u = [,, 2 t be an element of the range of T. Find the uniquely determined element v in the domain of T, such that T (v) = u, using A = Consider the following matrix: A = (a) Find the null space of A Setting x 2 = s, x 4 = s 2, x 5 = s 3, we get by backward substitution null(a) = span{ 3 5 }. (b) Find a basis of null(a), that contains the vector v = [5,,, 3, 2 t , 2 which shows, that the first, second and third columns are pivot columns, so that we take the respective vectors and conclude that { 3,, 3 } 2 is a basis as asked for. (c) What is the nullity of A? What is the rank of A? The basis in (b) has three elements, so nullity(a) = 3. By the rank-nullity theorem, we therefore have rank(a) = 2.

6 4. Consider the following matrix: (a) Determine the characteristic polynomial χ A of A. Show all your work! (Key: The characteristic polynomial is χ A = λ 3 2λ 2 λ.) A λi = λ 3 2 λ λ (b) Determine the eigenvalues for A. = λ(2 λ)(4+λ) 9 9+3(2 λ)+9( λ)+3(4+λ) = λ3 λ 2 λ. χ A = λ(λ 2 + 2λ + ) = λ(λ + ) 2 = λ = or λ =. (c) Compute the eigenspace for the eigenvalue that has multiplicity 2 in χ A. What is the dimension of this eigenspace? A ( )I = ( ) 3 2 ( ) ( ) 3 = Setting x 2 = s, x 3 = s 2, we get x = 3s + s 2. We therefore have as the eigenspace 3 E = span{, }. The dimension of this eigenspace is 2. (d) What is the only possible dimension of the eigenspace with eigenvalue λ =? Answer this question with the help of χ A and justify your answer. The multiplicity of as a root of χ A is. By Theorem 6.., the dimension of E must be less than or equal to, hence it is equal to.

7 5. Let S = span{s = 2, s 2 = 6 }. (a) Find an orthogonal basis using Gram-Schmidt. Following Gram-Schmidt, we get u = 2 u 2 = = 3 4 and thus, { 2, as an orthogonal basis of S. (b) Find a basis for S. We need to determine the following nullspace. [ } [ 6 4 Setting x 3 = s, we get x 2 = s and x 4 = 8 s. Hence, 4 8 S = span{ }. 4 (c) Compute the norm s 2 of s 2. s 2 = = 38. (d) What is the exact norm of s 2 s 2? s 2 s 2 = s 2 s 2 =.

8 6. Suppose that A is a (7, 7)-matrix. (a) What is the maximum possible value for rank(a)? The maximum possible value for the dimension of the col-/or rowspace is 7. (b) What is the minimum possible value for nullity(a)? By the rank-nullity theorem, we have a minimum possible nullity of. (c) Suppose that dim(col(a)) = 5. What is nullity(a)? The rank-nullity theorem we get nullity(a) = 2 7. Let A be a (2,2)-matrix, which has no eigenvalue over R. What is a possible characteristic polynomial for A in this case? A possible characteristic polynomial without a root over R is χ A = λ 2 +, for example. 8. Show that if u, u 2 are both orthogonal to v, then u + u 2 is also orthogonal to v. (u + u 2 )v = u u + u 2 v = + =, hence the desired orthogonality. 9. Give examples for [ infinitely many different subspaces of dimension one in R 2. For example, span{ } for n N are examples of infinitely many one dimensional n subspaces of R 2.. Calculate ([ ) 2 +. ([ [ 2 2 ) 2 [ = [ [ =

9 [ [ [ 4. (a) Let B = {, } be a basis for R and let u = be a vector 4 represented with respect to the standard matrix. What is the coordinate vector of x with respect to B. The change-of-base-matrix is so U = [ 4 3, U = [ Hence, u B = [ [ = 2 4 [ 4 (b) Find the missing vector v 2 in the basis B 2 = { Let the missing vector be [ v. The change-of-base formula gives [ 4 v v 2 v 2 [ 3 This gives a system with two linear equations = 4 + 3v =. [ [ 2 2, v 2 } such that + 3v 2 =, [ [ 4 with v = and v 2 = as solution. Hence B 2 = {, 2. Find the values for h such that A is not invertible, where 4 h A = h 3. h det(a) = h 3 4h = h(h 2)(h + 2). [ 3 B 2 = [. } is the desired basis. In order for A not to be invertible, the determinant must be zero. Hence h =, 2, are the values such that A is not invertible.

10 3. (a) Let A = Show without determining the nullspace of A that the vector [, 5,,,,,, t is an element of the nullspace of A. If v is in null(a), satisfies by definition Av =. We therefore check that = (b) Show that the vectors u, v R n are orthogonal, if and only if the vector v is an element of the nullspace of u t, when u t is considered as an (n, )-matrix. u, v are orthogonal, if and only if u v = if and only if u t v = [, if and only if v is in the nullspace of u t..